Question 1
Why is a plant kept in the dark for 48 hours before this experiment?
The right answer is the one marked above.
To remove all the starch already in the leaves. In the dark no new starch is made, while the starch already there is converted to sucrose and exported through the phloem.
Without this step, a leaf that turned blue-black might have made that starch last week. Destarching is what turns "there is starch here" into "starch was made here, today".